VATICAN CITY - Pope Francis urged South American bishops on Monday to speak "courageously" at a high-profile meeting on the Amazon, where the shortage of priests is so acute that the Vatican is considering ordaining married men and giving women official church ministries.

Francis opened the work of the three-week synod, or meeting of bishops, after indigenous leaders, missionary groups and a handful of bishops chanted and performed native dances in front of the main altar of St. Peter's Basilica.

Led in procession by the pope, the bishops then headed to the synod hall to chart new ways for the Catholic Church to better minister to remote indigenous communities and care for the rainforest they call home.

Among the most contentious proposals on the agenda is whether married elders could be ordained priests, a potentially revolutionary change in church tradition given Roman rite Catholic priests take a vow of celibacy.

The proposal is on the table because indigenous Catholics in remote parts of the Amazon can go months without seeing a priest or receiving the sacraments, threatening the very future of the church and its centuries-old mission to spread the faith in the region.

Another proposal calls for bishops to identify new "official ministries" for women, though priestly ordination for them is off the table.

Cardinal Claudio Hummes, the retired archbishop of Sao Paulo and the lead organizer of the synod, said the priest shortage had led to an "almost total absence of the Eucharist and other sacraments essential for daily Christian life."

"It will be necessary to define new paths for the future," he said, calling the proposal for married priests and ministries for women one of the six "core issues" that the synod bishops must address.

"The church lives on the Eucharist, and the Eucharist is the foundation of the church," he said, citing St. John Paul II.

Francis opened the meeting by extolling native cultures and urging bishops to respect their histories and traditions as they discern ways to better spread the faith.

History's first Latin American pope has long held enormous respect for indigenous peoples, and denounced how they are exploited, marginalized and treated as second-class citizens and "barbarians" by governments and corporations that extract timber, gold and other natural resources from their homes.

Speaking in his native Spanish, Francis told the bishops how upset he became when he heard a snide comment about the feathered headdress worn by an indigenous man at Mass on Sunday opening the synod.

"Tell me, what is the difference between having feathers on your head and the three-cornered hat worn by some in our dicasteries?" he said to applause, referring to the three-pointed red birettas worn by cardinals.

Francis urged the bishops to use the three weeks to pray, listen, discern and speak without fear.

"Speak with courage," he said. "Even if you are ashamed, say what you feel."

The synod is opening with global attention newly focused on the forest fires that are devouring the Amazon, which scientists say is a crucial bulwark against global warming. It also comes at a fraught time in Francis' six-year papacy, with conservative opposition to his ecological agenda on the rise.

Francis' traditionalist critics, including a handful of cardinals, have called the proposals in the synod working document "heretical" and an invitation to a "pagan" religion that idolizes nature rather than God.

To that criticism, Hummes denounced Catholic "traditionalism" that is stuck in the past versus the church's true tradition, which always looks forward.

"The church cannot remain inactive within her own closed circle, focused on herself, surrounded by protective walls and even less can she look nostalgically to the past," he said. "The Church needs to throw open her doors, knock down the walls surrounding her and build bridges."

In keeping with the meeting's environmental message, the synod organizers themselves are taking measures to reduce their own carbon footprint.

Organizer Cardinal Lorenzo Baldisseri told the bishops there would be no plastic cups or utensils at the meeting, that synod swag such as bags and pens were biodegradable, and that the emissions spent to get more than 200 bishops and indigenous from South America to Rome - estimated at 572,809 kilograms of carbon dioxide - would be offset with the purchase of 50 hectares of new growth forest in the Amazon.